Spermtaogenesis is a complex, highly ordered process of cell division and differentiation by which spermatogonial stem cells give rise to large numbers of mature spermatozoa. It is an essential determinant of fertility in all mammals, playing a fundamental role in the perpetuation of the human species, maintaining its genetic diversity and driving evolution. Successful completion depends on a precisely orchestrated cascade of developmentally regulated genes controlling and coordinating mitotic spermatogonial cell proliferation, meiotic chromosomal reduction divisions and finally morphological differentiation to produce mature sperm. This proposal is designed to identify key molecular mechanisms underlying the early stem cell renewal and proliferation phase of spermatogenesis. Very little is known about this process but clearly, any failure to regenerate spermatogonial stem cells in sufficient quantities will lead to a complete cessation of spermatogenesis. The Juvenile Spermatogonial Depletion (jsd) spontaneous mouse mutant will be used as a model system. The phenotype observed in the developing jsd/jsd male is one of a single wave of spermatogenesis during early puberty, followed by failed germ cell proliferation during late puberty and adulthood. Adults are azoospermic with very small testis, good numbers of Sertoli cells but very few type A spermatogonia. This indicates that absence of spermatogenesis is due to an underlying failure of the spermatogonial stem cell renewal process. The data accumulated to date are consistent with the hypothesis that a deficiency in proliferative signals from Sertoli cells, needed for mitotic stem cell renewal, causes the phenotype. jsd represents a unique, non-pleiotropic animal model for analysing the regulation of early germ cell proliferation and also provides a firm genetic basis for the study of certain types of idiopathic infertility in man. The specific aims of this project are designed to map, clone and initiate a characterization of the gene underlying the jsd phenotype. The contribution of the human JSD homologue to male infertility will be assessed using a large resource of well characterized infertile men available in our laboratory. This is a particularly important aspect, for what is seen in adult testis sections of certain classes of infertile men, may actually represent the end result of a failure of stem cell renewal - a process that started much earlier in the juvenile stage and may well be amenable to treatment.